The chemical composition of two stars in WLM have been determined from high quality UVES data obtained at the VLT UT2 Based on observations collected at the European Southern Observatory , proposal number 65.N-0375 .
The model atmospheres analysis shows that they have the same metallicity , [ Fe/H ] = - 0.38 \pm 0.20 ( \pm 0.29 ) In this paper , we adopt the standard notation [ X /H ] = log ( X /H ) - log ( X /H ) _ { \odot } .
Also , abundances shall be reported with two uncertainties : the first is the line-to-line scatter , and the second ( in parentheses and italics ) is an estimate of the systematic error due to uncertainties in the atmospheric parameters . .
Reliable magnesium abundances are determined from several lines of two ionization states in both stars resulting in [ Mg/Fe ] = - 0.24 \pm 0.16 ( \pm 0.28 ) .
This result suggests that the [ \alpha ( Mg ) /Fe ] ratio in WLM may be suppressed relative to solar abundances ( also supported by differential abundances relative to similar stars in NGC 6822 and the SMC ) .
The absolute Mg abundance , [ Mg/H ] = - 0.62 is high relative to what is expected from the nebulae though , where two independent spectroscopic analyses of the H ii regions in WLM yield [ O/H ] = - 0.89 .
Intriguingly , the oxygen abundance determined from the O i \lambda 6158 feature in one WLM star is [ O/H ] = - 0.21 \pm 0.10 ( \pm 0.05 ) , corresponding to five times higher than the nebular oxygen abundance .
This is the first time that a significant difference between stellar and nebular oxygen abundances has been found , and presently , there is no simple explanation for this difference .
The two stars are massive supergiants with distances that clearly place them in WLM .
They are young ( \leq 10 Myr ) and should have a similar composition to the ISM .
Additionally , differential abundances suggest that the O/Fe ratio in the WLM star is consistent with similar stars in NGC6822 and the SMC , galaxies where the average stellar oxygen abundances are in excellent agreement with the nebular results .
If the stellar abundances reflect the true composition of WLM , then this galaxy lies well above the metallicity-luminosity relationship for dwarf irregular galaxies .
It also suggests that WLM is more chemically evolved than currently interpreted from its color-magnitude diagram .
The similarities between the stars in WLM and NGC6822 suggest that these two galaxies may have had similar star formation histories .